I have a revised USB to serial mini adapter in an enclosed power box to a board to my arduino pro mini.I thought it would be cool to connect mounted LED's (2 mm)to the TX and RX, can i run it off of the lines directly, or would I have to feed the signal to a transistor and LED, or is not powerful enough to do either. I noticed that the built on LED's connect to different pins of the adapter according to the schematics, what's up with that?

Yes, can run from the lines directly, use 1K or higher resistor so there is little load on the lines.Measure them with no transfer occurring - if they sit at a high level, you may want to add an inverter so the LEDs only light up when a transfer is occurring.The built-on LEDs are driven from IO pins on the USB/Serial chip.